1. Field of the Invention
The invention relates to EFM/EFM+ encoding, and in particular to a EFM/EFM+ encoder and a method thereof.
2. Description of the Related Art
In optical disc recording, data is transferred thereto in 8-bit form, known as a data symbol. The disc system then generates header and synchronization information, control bytes, identification data, and copyright management information, processes data including data scrambling and error correction code generation on the data symbols, and modulates the data symbols using EFM (Eight-to-Fourteen Modulation) for CD and EFM+ (Eight-to-Fourteen Modulation Plus) for DVD to produce modulated bits and channel bits for recoding.
Channel bits are typically transmitted in a Non Return to Zero Inverted (NRZI) format, comprising two possible states at one of which the channel bit remains until a binary one occurs in the modulated bits, i.e., each channel bit may either at +1 or −1 state. A channel bit is the minimum recording unit T on CDs and DVDs, referred to as Run-length Limited (RLL) code, meaning the number of consecutive binary zeros in the encoded bit pattern must be at least as large as a specified non-zero minimum and not exceeding a specified maximum. For example, CDs typically employ a code specified as 3-11T RLL, meaning the number of consecutive zeros in the encoded bit pattern must be at least 2 and not exceeding 10.
In a multimedia playback system, a data slicer typically deploys the DC value of the channel bits as the reference to determine the states of NRZI format channel bits. Since each channel bit is either in +1 or −1 state, it is crucial to ensure the DC value of the channel bits is close to 0, or a DC-free value, to determine each channel bit accurately. A sum of the state of consecutive channel bits is referred to as A Digital Sum Value (DSV), indicating the DC value of the channel bit. Any DSV exceeding the specified maximum is likely to cause data read errors or problems in reading data on CDs and DVDs.
In EFM encoding for CDs, an EFM encoder takes each 8-bit data symbol as an index into a conversion table of channel bit patterns to converts to 14-bit channel bit sequence. The modulated 14-bit channel bits are referred to as a codeword. Each codeword satisfies the 3-11T RLL constraint. A 3-bit merging bit is determined to join two consecutive codewords so that the concatenated channel bits sequence may not violate the 3-11T RLL constraint. With appropriate merging bits the resulting channel bit sequence can meet both 3-11T RLL and DSV requirements. However, inappropriate choice of merging bits results in large DSV, leading to inaccurate data reading.
FIGS. 1a and b show an example of a conventional channel bit sequence that produces divergent DSV. When EFM is used to modulate a special data pattern of {0x9a, 0xb9, 0x9a, 0xb9, 0x9a, 0xb9 . . . } (hexadecimal form), it is noted that the absolute value of DSV generated from the channel bits after modulation will cumulatively increase and cannot be controlled via the standard EFM modulation. If the value of DSV cannot be kept small, the excessively large DSV of the recorded data results in the data slicer, conventionally used to retrieve the binary signal from the analog signal detected on optical discs, being unable to function correctly, and the data readout from the disc is erroneous. Moreover, the large DSV variance implies that the EFM signal is no longer DC-free, and the low-frequency components of the EFM signal interfere with the related servo control signal of the optical disc system.
Thus a need exists for an EFM/EFM+ encoder and method of inhibiting copying unauthorized data on an optical recording medium.